Single Photon Emission Computed Tomography (SPECT) and Positron Emission Tomography (PET) are the most common devices in nuclear medicine imaging, which is used to provide important information of, e.g., heart function or tumor growth and location. A complicating factor in SPECT and PET procedures is that breathing of the patients results in motion of the imaged objects.
In contrast to CT where a beam is sent through the body and detectors measure the line integral on the opposite side, the signal in SPECT and PET comes from a radioactive tracer being inside the body. Thus, the exact location of the source of the signal is a-priori not known, so that it is also unknown how much of the signal was absorbed by other structures on the way to the detector. However, when morphological and material information about the acquired region is additionally provided, absorption correction can be performed on the SPECT or PET image. The necessary information is typically provided by additionally acquiring MRI or CT images, where MRI is attractive due to the fact that it does not expose the subject to ionizing radiation while CT has the advantage that attenuation properties can be directly derived from the data. The corresponding maps that contain the absorption information in the morphological image are called attenuation maps.
Since the patient breathes freely during image acquisition, the obtained SPECT and PET images contain severe motion artifacts. During respiratory motion many organs are at a largely different location with most of them also being significantly deformed. Due to this respiratory motion, it is very difficult to provide an accurate attenuation map for providing the correct morphological information for the nuclear images. In this context, it has to be noted that breath-holding is not an option due to the long acquisition time of SPECT and PET scans.
In the past, some solutions have been proposed for dealing with the problems caused by motion of the object in nuclear imaging. The international patent application published as WO 03/107275 describes the use of a motion model in combination with acquired CT images in order to generate a subject-specific physiological model. The subject specific model is used for the registration of the CT images and PET images. The use of the motion model facilitates the registration of spatially aligned images. Although this published patent application does provide a method for accurate spatial alignment of images from different imaging devices, the quality of the registered nuclear images is still strongly affected by the respiratory motions of the patient.